Those skilled in the arts of antenna arrays and beamformers know that antennas are transducers which transduce electromagnetic energy between unguided- and guided-wave forms. More particularly, the unguided form of electromagnetic energy is that propagating in “free space,” while guided electromagnetic energy follows a defined path established by a “transmission line” of some sort. Transmission lines include coaxial cables, rectangular and circular conductive waveguides, dielectric paths, and the like. Antennas are totally reciprocal devices, which have the same beam characteristics in both transmission and reception modes. For historic reasons, the guided-wave port of an antenna is termed a “feed” port, regardless of whether the antenna operates in transmission or reception. The beam characteristics of an antenna are established, in part, by the size of the radiating (or receiving) portions of the antenna relative to the wavelength. Small antennas make for broad or nondirective beams, and large antennas make for small, narrow or directive beams. When more directivity (narrower beamwidth) is desired than can be achieved from a single antenna, several antennas may be grouped together into an “array” and fed together in a phase-controlled manner, to generate the beam characteristics of an antenna larger than that of any single antenna element. The structures which control the apportionment of power to (or from) the antenna elements are termed “beamformers,” and a beamformer includes a beam port and a plurality of antenna element ports. In a transmit mode, the signal to be transmitted is applied to the beam port and is distributed by the beamformer to the various element ports. In the receive mode, the unguided electromagnetic signals received by the antenna elements and coupled in guided form to the element ports are combined to produce a beam signal at the beam port of the beamformer. A salient advantage of sophisticated beamformers is that they may include a plurality of beam ports, each of which distributes the electromagnetic energy in such a fashion that different beams may be generated simultaneously.
Radar systems often use multiple antenna beams for tracking of disparate targets, and sometimes for tracking single targets. One scheme for use of multiple beams involves monopulse techniques, in which angle tracking information is obtained from multiple beams, ideally with but a single transmitted pulse. Monopulse operation is accomplished by generating two, or more usually three, antenna beams, so that the simultaneously received echoes from the multiple beams can be compared. The usual monopulse beams are a sum (Z) beam, and azimuth (Az) and elevation (El) difference (A) beams. Monopulse systems are described in many publications, as for example in U.S. Pat. No. 5,017,927 issued May 21, 1991 in the name of Agrawal et al. Agrawal et al. in one arrangement uses three separate beamformers, namely Σ, Az Δ, and El Δ beamformers, to generate the three different beams. These beamformers can be manifested in an array of a plurality of elevation Σ, Az Δ, and El Δ column beamformers which connect to the antenna elements, and an array of azimuth Σ, Az Δ, and El Δ row beamformers, which connect the Σ, Az Δ, and El Δ ports to the column beamformers.
FIG. 1 is a representation of a prior-art array antenna as described in the above-mentioned Agrawal et al. patent. As described therein in FIG. 1, radar system 10 includes an antenna array 12 including individual antennas or antenna elements 141, 142, 143, . . . 14N-2, 14N-1, and 124N arrayed in a column designated 161. Other columns 162, 163 . . . 16N are illustrated in a general manner as being located behind column 161, so as to form a two-dimensional rectangular array of antenna elements.
Each antenna element 141, 142 . . . 14N of columns 161, 162, . . . 16N of antenna array 12 of FIG. 1 is associated with a phase shifter 18. For example, elemental antenna 141 of column 161 is associated with a phase shifter 181. Similarly, each of the elemental antennas 142, 143 . . . 14N of column 161 are associated with a phase shifter 182, 183 . . . 18N. As also illustrated in FIG. 1, phase shifter 181 has an output transmission line (cable) 201 which, together with output cable 20N of phase shifter 18N of column 161, is connected to a sum-and-difference hybrid circuit 221. Each of cables 201 and 20N is connected to a separate input port (input) of hybrid circuit 221. It will be noted that phase shifters 181 and 18N are associated with elemental antennas 141 and 14N, the first and last (top and bottom) antenna elements of column 161. Similarly, the output of phase shifter 182 is coupled by way of a cable 202 to a second sum-and-difference hybrid splitter 222, together with the output from phase shifter 18N-1, coupled by way of a cable 20N-1. Phase shifter 182 is associated with antenna element 142, the second antenna element, and phase shifter 18N-1 is associated with penultimate antenna element 14N-2. A third sum-and-difference hybrid combining arrangement 223 receives inputs from the third antenna element 143 and its phase shifter 183 by way of cable 203, and from antepenultimate antenna element 14N-2 and its phase shifter 18N-2 by way of cable 20N-2, respectively. It can be seen that the outputs of the antenna elements of column 161 and their phase shifters are taken in pairs symmetrically disposed above and below the center of column 161, and the antenna outputs are combined in an array of sum-and-difference hybrids. The combination or array of sum-and-difference hybrids 22 associated with column 161 is designated 241.
Each of the other columns of FIG. 1, such as column 162, 163 . . . 16N, includes (not illustrated) its own column array of antenna elements 14 and phase shifters 18, each of which is associated with an antenna 14. Each of the other columns is also associated with an array 24 (not illustrated) of sum-and-difference hybrids 22. Only antenna array column 16N is illustrated in FIG. 1 as being connected by cables 20 to its associated sum-and-difference hybrid array 24N.
In the arrangement of FIG. 1, the sum output produced at the upper output of hybrid 221 of hybrid array 241, is coupled by way of a cable 261 to an input of a sum combiner or beamformer 301. Similarly, the upper or sum (Σ) outputs of sum-and-difference hybrids 222 and 223, and all the other hybrids (not illustrated) of hybrid array 241, are coupled by a cable 26 to sum combiner 301, which combines the sum signals, and which couples the combined sum signals to a single output cable 341. Similarly, the difference (Δ) output ports of sum-and-difference hybrids 221, 222, 223, . . . 22n/2 of hybrid array 241 of FIG. 1 are each connected by way of a transmission line 28 to separate inputs of a difference combiner or beamformer 321. Thus, the Δ (lower) output port of hybrid 221 is connected by way of a cable 281 to a first input of Δ combiner 321, the a output port of hybrid 222 is coupled by way of a cable 282 to a second input of Δ combiner 321, and the Δ output port of hybrid 223 is coupled by cable 283 to a third input of Δ combiner 321. All the other hybrids (not illustrated) of hybrid array 241 have their Δ output ports coupled to a Δ combiner 321 in a similar manner. Combiner 321 combines the ‘signals and couples their sum to an output cable 36’.
Each of the other hybrid arrays 242 . . . 24M (only 24M illustrated) of FIG. 1 are connected to an associated pair of sum and difference combiners or beamformers in the same manner. The Mth hybrid array, namely 24M, is illustrated in FIG. 1, together with some of its cables 20, and also with some connection 26 to last column E combiner 30M. As so far described, all the columns 161 through 16M ultimately produce a sum signal from a column sum combiner 30 on a cable 34, and a difference signal from a column Δ combiner 32 on a cable 36. Thus, there are M cables 34, and M cables 36, one for each column 16. Elemental phase shifters 18 can be adjusted so that the input signals to column Σ combiners 30 add in-phase for a desired antenna beam pointing direction. Difference signals to column Δ combiner 32 will add in-phase only if cable pairs 26N and 28N are phase matched for all N, provided that the Σ and Δ combiners for each column have identical topologies. First cable 341 and last cable 34M from sum combiners 301 and 30M, respectively, are coupled to individual inputs of a sum-and-difference hybrid designated 381. The outputs from the second (302) and penultimate (30M-1) combiners (not illustrated) are coupled over cables 342 and 34N-1 to separate input ports of a second sum-and-difference hybrid 382. Similarly the third (303) and antepenultimate (30M-2) sum combiners 30 (not illustrated) have their outputs coupled by way of cables 343 and 34M-2, respectively, to a sum-and-difference hybrid 383. Other sum-and-difference hybrids (not illustrated) together with hybrids 381, 382, and 383, form an array 40M of sum-and-difference hybrids. Each hybrid of array 40M receives inputs from a pair of column sum combiners 30 associated with a pair of columns 16, the columns of which are symmetrically disposed to the left and right of the center of array 12.
The sum outputs of the hybrids of hybrid array 40M of FIG. 1 are each separately coupled by way of a cable 44 to a separate input of an azimuth sum combiner 48. For example, hybrid 381 has its E output connected by way of a cable 441 to an input of azimuth combiner 48, hybrid 382 has its E output connected by a cable 442 to another input of azimuth combiner 48, and hybrid 383 has its Σ output connected by way of a cable 443 to a third input of azimuth sum combiner 48. Azimuth sum combiner combines the Σ signals and produces the combined Σ signal on a cable 50 for application to a processing and display unit illustrated as 70. The Δ outputs of each of sum-and-difference hybrids 38 of hybrid array 40 of FIG. 1 are each separately coupled by way of a cable 46 to separate inputs of an azimuth Δ combiner 52. For example, the Δ output of hybrid 381 is connected by way of a cable 461 to an input of azimuth Δ combiner 52, the Δ output of hybrid 382 is connected to a second input of azimuth Δ combiner 52 by way of a cable 462, and the Δ output of hybrid 383 is connected by way of a cable 463 to yet another input of combiner 52. Combiner 52 combines the Δ signals and applies the combined signals over a cable 54 to processing and display unit 70 of radar unit 10. Another array 41 of sum-and-difference hybrids, each of which is designated as 42 in FIG. 1, is coupled to the array of M column Δ combiners 32 (only combiner 321 is illustrated), in much the same fashion that array 40 of hybrids 38 is coupled to an array of M sum combiners 30. For example, sum-and-difference hybrid 421 receives inputs by way of cables 361 and 36M from first and last column A combiners 321 and 32M (not illustrated). Sum-and-difference hybrid 422 is connected by way of cable 362 and 36M-1 to the second and penultimate column Δ combiner 32 (not illustrated), and hybrid 423 has its inputs connected by way of cables 363 and 36M-2 to the third and antepenultimate column Δ combiners 32. Other hybrids 42 of array 41 are connected to other pairs of combiners symmetrically disposed to the left and right about the center of array 12.
The sum outputs of each of sum-and-difference hybrids 42 of array 41 of FIG. 1 are coupled by way of separate cables 56 to separate inputs of an elevation Δ combiner 62. For example, hybrid 421 has its sum output connected by way of a cable 561 to a first input of combiner 62, and the sum outputs of hybrids 422 and 423 are connected by separate cables 562 and 563, respectively, to other inputs of elevation Δ combiner 62. Elevation Δ combiner 62 combines the column Δ signals to produce an elevation Δ signal on a cable 64 for application to processing and display unit 70. The difference (Δ) outputs of sum-and-difference hybrids 42 of hybrid array 41 of FIG. 1 are not used and are terminated. For example, the Δ output of hybrid 421 is coupled by way of cable 581 to a termination 601, and the Δ outputs of hybrids 422 and 422 are coupled by cables 582 and 583 to terminations 602 and 603, respectively.
A transmitter 72 associated with radar system 10 of FIG. 1 is coupled to processing and display unit 70 for timing the signals, for providing appropriate demodulation reference signals, and for other purposes. Also, a transmitter signal is applied to cable 50 of azimuth sum combiner 48, as suggested by dotted lines 74 within processing and display unit 70. The transmitter signals are coupled through azimuth combiner 48 and back through the arrays of hybrids and combiners, which in the context of transmission may act as splitters, to ultimately produce signals at antenna elements 14, which signals are phased in a manner appropriate for directing radiation in a particular direction.
The complexity of the beamforming arrangement of FIG. 1 is apparent. Additional complexity arises because of the amplitude weighting of the signals relative to each other in each column 16, and from column to column, in order to achieve the appropriate beam sidelobe levels for both elevation and azimuth beams. Even if phase shifters 18 are set correctly, assuming equal phase signals arriving at the phase shifters, cumulative phase errors through the combiners and hybrid arrays may adversely affect the performance. In this regard, it should be noted that the actual physical lengths of interconnecting cables such as 201, 202 . . . 20M must be nearly equal for wide bandwidth signals, and some cables such as 26N and 28N must have the same electrical length as well, even though the distances over which the signals must be carried may be less than the physical lengths. This in turn tends to create a problem relating to excess cable lengths associated with the shorter paths, which excess cable lengths must be stored out of the way.
FIG. 2a is a simplified block diagram of a monopulse antenna array arrangement as described by Agrawal et al. Elements of FIG. 2a corresponding to those of FIG. 1 are designated by the same reference numerals. Array 12 of FIG. 2a includes a plurality of columns 2161, 2162, 2163 . . . 216M, corresponding generally to columns 16 of FIG. 1. Each column 216 of FIG. 2a includes a vertical array of N antenna elements 14, such as 141, 142, 143 . . . 14N-2, 14N-1, and 14N. Each antenna element 14 of each column 216 is associated with a transmit-receive processor or module (TR Proc). Thus, antenna element 141 of column 2161 is associated with a TR Proc 2181, elemental antenna 142 is associated with TR Proc 2182, and antenna 14N is associated with TR Proc 218N. Structurally, all TR Procs 218 are identical, although their adjustable portions (phase shifters, attenuators and/or switches) may be set differently.
As illustrated in FIG. 2a, each transmit-receive processor 218 has three outputs, designated 219, 220, and 221. For simplicity, the outputs of the TR processors are designated by the same reference numerals as that of the cables to which they are attached. Thus, outputs 2191, 2201 and 2211 of TR Proc 2181 of column 2161 are connected to cables 2191, 2201 and 2211, respectively. In a similar manner, the three outputs of TR Proc 2182 of column 2161 are connected to cables 2192, 2202 and 2212, respectively. The three outputs of TR Proc 218N of column 2161 are separately connected to cables 219N, 220N and 221N. As illustrated in FIG. 2a, the topmost or first TR processor 2181 of column 2162 is seen to be associated with output cables 2191, 2201, and 2211. In column 216M, TR processor 2181 is associated with cables 2191, 2201, and 2211. As in the case of FIG. 1, of course, all the columns 2162 . . . 216N are identical to column 2161.
The arrangement of FIG. 2a includes a Σ beamformer 230, an azimuth Δ beamformer 229, and an elevation Δ beamformer 231. All the cables 219 connected to TR processors 218 of array 12 are gathered in rows and columns in azimuth Δ beamformer 229. For example, all the cables 2191 from TR processors 2181 of all M columns 216 are separately connected to separate inputs located along a top row of beamformer 229. Similarly, all the cables 2192 from all the M TR processors 2182 of all columns 216 of array 12 are gathered and connected to the second row of inputs (not illustrated, in FIG. 2a) of azimuth Δ beamformer 229.
FIG. 2b illustrates the connections of TR processors 218 of FIG. 2a to azimuth Δ beamformer 229 of FIG. 2a. In FIG. 2b, the connection face of beamformer 229 is seen in elevation view, with some of the inputs illustrated as dots. The connection face of beamformer 229 contains MXN input ports, one for each TR Proc 218, laid out as M columns and N rows. As can be seen, the upper row of inputs of beamformer 229 for columns 1, 2, 3 . . . M−2 μM−1, M are each connected to a cable 2191. The second row of connections of beamformer 229 is to cables 2192, and the bottommost row of connections on the connection face of beamformer 229 receives cables 219N.
Sum beamformer 230 of FIG. 2a is connected to receive cables 220 in a same manner in which beamformer 229 is arranged to receive cables 219. That is, the topmost row of the connection face (not illustrated) of sum beamformer 230 is connected to cables 2201 from all M columns. The second row is connected to cables 2202, and so forth, until the lowermost row is connected to all cables 220N from all M columns. Elevation Δ beamformer 231 is similarly connected to receive cables 221 from all TR Procs 218 of array 12. Azimuth Δ beamformer 229 of FIG. 2a collects all the signals provided over cables 219 to form an azimuth difference signal which is coupled out over a cable 54. In the context of a radar system, cable 54 may be connected to a processor and display unit as described in conjunction with FIG. 1. Similarly, sum beamformer 230 and elevation difference beamformer 231 combine the signals from cables 220 and 221, respectively, to produce combined signals on cables 50 and 64, respectively.
FIG. 3 illustrates one possible arrangement for interconnecting the transmit-receive processors 218 of the arrangement of FIG. 2, as set forth in the Agrawal et al. patent. In FIG. 3, elements corresponding to those of FIGS. 1 and 2 are designated by the same reference numerals. In FIG. 3, only column 216 and a portion of column 216M are illustrated. Each column of the array, including columns 2161 and 216M, is associated with three individual column beamformers designated 329, 330 and 331. In FIG. 3, azimuth Δ column beamformer 3291 is connected to receive cables 2191, and all other cables 2192, 219N of TR processors 2182-218N of column 216. Column 2161 sum beamformer 3301 receives inputs from cables 2201, 2202, 2202, . . . 220N-2, 220N-1, and 220N. Elevation Δ column beamformer 3311 is connected to receive cable 2211 from TR processor 2181 of column 2161 and cables 2212.. 221N from the remaining TR processors 218 of column 2161. Thus, column 2161, and all other columns 216 of array 12, is or are associated with three column beamformers, one for sum, one for azimuth Δ and the other for elevation Δ. Thus, cables 2201, 2202, 2203 . . . connect from TR processors 2181, 2182, 2183 of column 216M to sum column beamformer 330M. Although not illustrated in FIG. 3, column M azimuth difference beamformer 329M is connected to cables 2191, 2192 . . . from the TR processors of column 216M, and column M elevation Δ beamformer 331M is connected to cables 2211, 2212 . . . 221N from the TR processors 218 of column 216M. Each column beamformer 3291-329M of FIG. 3 produces a signal on an output cable 3491-349M. All cables 3491 . . . 349M are connected to corresponding inputs of an array azimuth Δ beamformer 339, which combines the column signals to produce an array azimuth Δ signal on a cable 54. Similarly, elevation Δ column beamformers 3311 . . . 331M each produce a combined output on a corresponding cable 3511 . . . 351M, which are all connected to an array elevation Δ beamformer 341, which combines the signals to produce a combined elevation Δ signal on cable 64. Finally, each sum column beamformer 330 . . . 330M combines its signals to produce a combined signal on a corresponding cable 3501 . . . 350M. All cables 3501 . . . 350M are connected to corresponding inputs of an array sum beamformer 340, which combines the signals to produce a combined sum signal on a cable 50. Array Σ beamformer 340 of FIG. 3, together with M associated column Σ beamformers 330, may be considered equivalent to sum beamformer 230 of FIG. 2a. Similarly, AZ Δ beamformer 229 of FIG. 2a corresponds to the combination of azimuth Δ beamformer 339 of FIG. 3 with a plurality equal to M of column AZ A beamformers 329. Elevation Δ beamformer 231 of FIG. 2a corresponds to the combination of elevation Δ beamformer 341 of FIG. 3 with all M of the column EL Δ beamformers 331.
FIG. 4 is a simplified block diagram of a transmit-receive processor 218 which may be used in the arrangements of FIG. 2 or 3. Elements of FIG. 4 corresponding to those of FIGS. 2 and 3 are designated by the same reference numerals. A port 410 at the right of FIG. 4 is available for connection to the associated antenna element 14. A transmit amplifier designated generally as 412 includes a power amplifier illustrated as 414 and a driver amplifier 416. Broadcasting of harmonics of the transmitted signal is reduced by a harmonic filter 418. Also in FIG. 4, a receive amplifier arrangement designated generally as 420 includes a low noise amplifier (LNA) 422 preceded by an amplitude limiter 424. Transmitted signals are transmitted from transmit amplifier 412 to port 410, and to the associated antenna element 14 (not illustrated) by way of a circulator 426, and signals received by the antenna element are coupled from port 410 to receive amplifier arrangement 420, also by way of circulator 426, which provides isolation between transmit amplifier arrangement 412 and receive amplifier arrangement 420. A phase shifter 428 has its output connected to a variable gain amplifier 430, which may be used to compensate for changes in the loss of the phase shifter when the phase shifter is controlled to assume various values of phase shift. A switching arrangement designated generally as 431 includes a first switch 432 including a common element illustrated as 434, and also including switch terminals 436 and 438. Mechanical switch symbols are used for purposes of explanation, but those skilled in the art know that solid-state equivalents may provide performance which may be superior. Common element 434 of switch 432 is connected to a port 220, which is the port which is connected to the sum combiners in the arrangements of FIGS. 2 and 3. Switching arrangement 431 includes a second switch 440, which has a common element connected to the output of variable gain amplifier 430, and switch terminals 442 and 444. Switch terminal 442 is connected to the input port of transmit driver 416. A receive post amplifier 447 is connected between switch terminal 444 of switch 440 and switch terminal 436 of switch 432. A third switch 446 of switching arrangement 431 includes a common element connected to the input port of phase shifter 428, a switch terminal connected to switch terminal 438 of switch 432, and a further terminal 448.
A coupling arrangement designated generally as 450 in FIG. 4 includes a one-to-N power divider 452, the input of which is coupled to the output of low noise amplifier 422, for dividing the amplified received signal into a plurality of portions. As illustrated in FIG. 4, the number of portions is three. A first portion is coupled by way of a path 454 to switch terminal 448 of switch 446. The other portions are described below. Each module 218 is also associated with or contains power conditioning and switching circuits illustrated as 456, and logic circuits illustrated as including an application-specific IC 458 controlled by external commands received from a port 460. Application-specific IC 458 addresses a programmable ROM (PROM) portion 461 of the logic circuit to generate commands for a control driver 462. Control driver 462 in turn commands the operation of switches 432, 440 and 446, the value of the phase shift provided by phase shifter 428, the magnitude of gain provided by variable gain amplifier 430, and other appropriate variable elements (not illustrated).
As so far described, the arrangement of FIG. 4, with the switches in the illustrated positions, is arranged to receive signals to be transmitted at input port 220, to pass those transmitted signals through the selected value of phase shift in phase shifter 428, and to apply the phase shifted signals to transmit amplifier arrangement 412 for amplification and for application through filter 418 and circulator 426 to the associated antenna element 14. The magnitude of the phase shift is selected by control arrangements associated with the control of the entire array, of which a particular module 218 is only a part. With switches 432, 440, and 446 of FIG. 4 in their alternate positions (not illustrated), signals received by antenna element 14 are coupled by way of port 410 and circulator 426 to limiter 424 and receive amplifier arrangement 420. Amplified received signals are coupled by way of power divider 452 and path 454, through switch 448 to the input of phase shifter 428, where they are phase shifted by the same phase shifter which provided phase shifting in the case of the transmit mode of operation. The phase shifted signals are again attenuated, and coupled by way of switch 440 (in its alternate position), through receive post amplifier 447 and, by way of switch 432 (in its alternate position), to port 220 for transmission therefrom to the sum combiner. This arrangement has the distinct advantage of using the combiner, phase shifter 428, and variable gain amplifier 430 for both transmit and receive operations.
According to a further aspect of the Agrawal et al. arrangement, the signals coupled to other beamformers, such as azimuth Δ beamformer 229 and elevation Δ beamformer 231 of FIG. 2a, are phase shifted by additional controllable phase shifters independent of phase shifter 428, which is used exclusively for the sum combiner or the sum beamformer. Thus, the value of phase shift provided by phase shifter 428 of FIG. 4 need not be a compromise. In the arrangement of FIG. 4, coupling arrangement 450 includes a further path designated generally as 464 between an output 465 of power divider 452 and a port 219. Path 464 includes a controllable phase shifter 466, a variable gain amplifier 468 and a further receive post amp 470. Variable gain amplifier 468 sets elemental gain for the desired array amplitude taper and resulting side lobe performance, and compensates for amplitude errors attributable to phase shifter 466. Port 219 is connected to the azimuth difference beamformer in the arrangement of FIG. 2. Coupling arrangement 450 also includes a further path designated generally as 472 between another output port 473 of power divider 452 and an output port 221. Path 472 includes a controllable phase shifter 474, an associated variable gain amplifier 476, and a further receive post amp 478. The output of post amplifier 478 is connected to output port 221. Coupling path 450 may include further paths connected to further output ports of power divider 452 for separately phase shifting and level setting signals intended for beamformers associated with monopulse beams other than Σ, AZΔ and ELΔ. Such additional paths are suggested by line 480 connected to an output port 479 of power divider 452.
As so far described, the arrangement of FIG. 2 using a TR module as illustrated and as described in conjunction with FIG. 4 has the salient advantage that the beamformers are not critical, and need not have connections thereto made in matched pairs equidistant from the center lines of the array. Thus, there may be a great saving in cable length and weight, and a reduction in the criticality of the phase through the various paths. Because the system can be tested, the phase shifts of the phase shifters of each TR module can be adjusted to optimize the phase shift through the particular path connected to that output port of that module. The performance in either transmission or reception can therefore be optimized separately for each of the Σ, AZΔ and ElΔ beams, and for any other monopulse beams. Within the beamformers, sums of nearby subarray signals can be made in any order, thereby providing a significant saving in the length and weight of cables. For the difference beams, an extra 180° phase shift can be added to the elemental outputs from selected half arrays. The variable gain amplifiers can be adjusted not only to correct amplitude errors within the beamformer but can provide the amplitude taper required to reduce the sidelobe level. The beamformers can be designed using standardized coupling values rather than coupling values which are customized to provide the desired amplitude weighting. At each operating frequency of the array, the phase shifters and variable gain amplifiers can be programmed with the phase and amplitude required to correct the errors which occur at that particular frequency, thus providing improved antenna performance over that achievable with prior art arrangements. Accordingly, the limiting factor in the performance of such an array is the ability to measure errors, together with the long-term stability of the equipment.
A disadvantage of the arrangement described in conjunction with FIGS. 2, 3 and 4 lies in a number of phase shifters and the complex control which may be required therefor. A compromise between the prior-art arrangement of a single phase shifter for multiple beamformers and the arrangement of FIGS. 2 through 4, having a single phase shifter for each beamformer of the array, may be the use of two phase shifters for three beamformers. FIG. 5 illustrates portions of the arrangement of FIG. 4, with a coupling circuit 450 which provides only a single additional phase shifter. Elements of FIG. 5 corresponding to those of FIG. 4 are designated by the same reference numerals. FIG. 5 differs from FIG. 4 in that low noise amplifier 422 has its output coupled to a two-way power divider 552. One output of power divider 552 is coupled to the cascade of a variable gain amplifier 530 and a controllable phase shifter 528. The output of the cascade is connected to an output port 519 and is available to the difference beamformers. A second output of power divider 552 is coupled by way of a path 554 to terminal 448 of switch 446. This provides a path by which received signals can return to the sum beamformer, as described in conjunction with FIG. 4.
FIG. 6 is a simplified block diagram of an array antenna using two-output TR Proc modules such as those of FIG. 5. Elements of FIG. 6 corresponding to those of other FIGURES are designated by the same reference numerals. In FIG. 6, array 12 includes a plurality of elemental antennas 14, each coupled to the radiator port of an associated TR Proc 518, each of which is similar to that of FIG. 5. The TR Procs 518 are arranged in columns designated 6161, 6162, . . . , 616M. Each column includes N TR Procs 518. The R processors 518 of FIG. 6 differ from those of FIG. 3 in having only two outputs. The lowermost output 519 from each TR processor 518 is coupled to an elevation difference beamformer illustrated in FIG. 6 as 231. This beamformer is similar to beamformer 231 of FIG. 2a, which combines the Δ El signals from N×M TR Procs 518 to produce a combined ΔEl signal on a conductor 64, and may be implemented in the form illustrated in FIG. 3.
The uppermost or Σ outputs from each TR Proc 5181, 5182, . . . , 518M of column 6161 of FIG. 6 are connected to inputs of a Σ column beamformer 3301, corresponding to a beamformer of FIG. 3. Beamformer 3301 combines the Σ outputs from the TR Procs of column 6161 and produces a combined signal on a cable 3501. The uppermost Σ outputs of the TR processors (not illustrated) of columns 6162, 6163, . . . 616M are each collected by a corresponding Σ beamformer 3302, 3303, . . . , 330M, to produce outputs on cables 3502, 3503, . . . , 350M. As so far described, the arrangement is generally similar to that of FIG. 3, but there is only a single output cable from each column Σ beamformer. These single cables carry signals from which both the Σ and Δ azimuth beams must be generated. The desired pair of beams are generated from the signal on the single cables by applying the signals on cables 3501, 3502, . . . , 350M of FIG. 6 to a further array 618 of TR Procs 518. As illustrated in FIG. 6, array 618 includes TR Procs 5181, 5182, 5183, . . . , 518M. TR Proc 518 of array 618 may be identical to TR Procs 518 of columns 616. Each TR Proc receives signals from a corresponding cable 350, for, in a receiving mode, dividing the signals into two portions and applying them to a sum beamformer 340 and to a Δ azimuth beamformer 339. This arrangement has the advantage of relative simplicity compared with the arrangement of FIG. 3, while maintaining the advantage of substantial control over the three antenna patterns.
In FIG. 7, elements corresponding to those of other FIGURES are designated by the same reference numerals. The arrangement of FIG. 7 is generally similar to that of FIG. 6, but includes the use of power dividers for feeding pairs of beamformers in order to generate closely-spaced beam pairs which track with changes in a single phase shifter. The arrangement of FIG. 7 includes a plurality of vertical column arrays 716, each of which includes a plurality of elemental antennas 14, a like plurality of TR processors 518, and a pair of power dividers or splitters 710A, 710B for each TR processor. Thus, each antenna 14 produces four output signals. For example, the first vertical column array 7161 of FIG. 7, which is the only array illustrated in any detail, includes antennas 141, 142, 143, . . . , 14N. Taking antenna 141 as being typical, it is connected to the input of a TR processor 5181. TR Proc 5181 is connected to a first output cable 6201 and a second output cable 5191. Cable 6201 is applied to the input of a power divider illustrated as a block 7101a, while cable 5191 is applied to a second power divider 7101b. Power divider 7101a divides the signal received over its input cable 6201 into two portions, one of which is coupled onto a cable 7121, and another portion which is coupled onto a second cable 7141. Similarly, signal coupled into power divider 7101b from cable 5191 is divided into two portions, the first of which is coupled on to a first cable 7161 and the second of which is coupled onto a second cable 7181. All the other connections within column array 716 are similar and are not discussed in detail. The three lower cables from each power divider set are individually coupled to three beamformers 731, 732 and 733. In particular, cable 7141 from power divider 7101a is connected to a beamformer 733 by a path (not illustrated), and the cables 714 from all the other elements of each column array 716 are coupled separately to an input of beamformer 733. The output of beamformer 733 appears on a cable 7505. Similarly, all cables 716 from all power dividers 710b are coupled to a beamformer 732, which has an output cable 7506 and all cables 718 from power dividers 710b of all column arrays 716 are coupled to a further beamformer 731, which has an output cable 7507; it should be noted that the signal applied over cables 716 to beamformer 732 and those applied over cable 718 to beamformer 731 have the same amplitude and phase, since they are replicas of one another. According to the aspect of the embodiment of Agrawal et al., a relative progressive phase shift is provided to the inputs of one of beamformers 731 and 732 so as to slightly offset the beams which they generate. With these slight phase offsets, which may be provided by printed line length differences built into the beamformers, two separate beams are generated, which track together during beam steering under the control of a single one of the phase shifters within each of the TR processors 518 of the column arrays 716. Similarly, a relative progressive phase shift is provided between the inputs of beamformers 732 and 733, to generate yet another beam pair.
Output cables 712 of power dividers 7108 are separately connected to inputs of column sum beamformers illustrated as 330 in FIG. 7. Thus, cables 7121, 7122, 7123, . . . , 712N of column 7161 are connected to separate inputs of sum beamformer 3301. Corresponding cables of column arrays 7162 are similarly coupled to a column beamformer 3302, etc. Each column beamformer 3301, 3302, . . . , 330M sums the signals applied thereto and couples them onto a single output cable 750. For example, sum beamformer 3301 sums signals onto a single cable 7501. The combined signal produced at each cable 750 is coupled to an input of an associated TR processor 518 of an array 718. Each TR processor of array 718 generates two output signals. Each array 718 also includes a further pair of power dividers 710a and 710b connected to the outputs of each TR processor. Thus, the combined signal produced on each cable 750 generates four outputs. Each power divider 710a of array 718 produces an output on a first output cable 790 and on a second cable 792. For example, power divider 7101a divides the signal applied to its input port into two portions, a first of which is coupled onto output cable 7901, and the other portion of which is coupled to output cable 7921. Similarly, the signals coupled into power divider 7101b are divided into two portions, the first of which is coupled onto cable 7941, and a second of which is coupled onto cable 7961. All cables 790 are coupled separately to separate inputs of a first sum beamformer 7401, all cables 792 are coupled separately to separate inputs of a second sum beamformer 7402, all cables 794 are coupled separately to inputs of a further sum beamformer 7403, and all cables 796 are coupled separately to separate inputs of a beamformer 7404. Each beamformer 740 sums together its input signals and couples them to a single output cable. For example, sum beamformer 7401 couples its output on to a cable 7501, beamformer 7402 couples its output to cable 7502, sum beamformer 7403 couples its output on to a single cable 7503, and sum beamformer 7404 couples its output on to a single cable 7504.
In operation of the arrangement of FIG. 7, beamformers 7401 and 7402 receive inputs which are common in amplitude and phase. Beamformers 7401 and 7402 are provided with a relative progressive phase shift of their inputs so that the beams which they form are angularly spaced by a small angle. Similarly, the inputs of beamformers 7403 and 7404 are provided with a relative phase shift so that they produce slightly different angular spacing of their beams. These angular spacings track during scanning, because the scanning is controlled by the same phase shifter of the phase shifters 518 of array 718 or of the arrays 716. It should also be noted that there may be a progressive phase shift between the inputs of all beamformers 740 and beamformers 731, 732 and 733.
FIG. 8 illustrates a beam pattern which might be generated by an arrangement such as that of FIG. 7. In FIG. 8, seven separate beams from outputs 7501, 7502, 7503, 7504, 7505, 7506, and 7507 are respectively represented by circles 810, 812, 814, 816, 818, 820 and 822. These circles may be considered, for example, to represent a plot of the 1.0 dB beam width points of the antenna pattern. As illustrated in FIG. 8, adjacent pairs of circles, or beams, are considered to scan together, with the central position of each pair being separately steered. As noted by Agrawal et al., various forms of signal processing may be performed on the formed beams for noise reduction, target enhancement and the like, before display, but are beyond the scope of the description. Agrawal et al. also note that other embodiments of the arrangements of Agrawal et al. will be apparent to those skilled in the art. For example, beamformers 231 in FIG. 6 and/or 731, 732 and/or 733 in FIG. 7 can be designed like those in FIG. 3, where some of the combining is done within the array columns and the balance is done with a one dimensional horizontal beamformer for each of the desired beams. As another example, arrays of antennas may have shapes which are other than rectangular, as for example circular, in which case it may be desirable to interpret the terms “column” and “row”, as used hereinbefore, as “ring” and “radial”, respectively, while other shapes may require other interpretations. It should be noted that even circular arrays may have the elemental antennas arranged in rows and columns. The transmission lines interconnecting the various portions of the described system may be formed as coaxial cables, or as printed circuit transmission lines, or as waveguides, or as separate conductors, depending upon the application. Parallel digital signals may be carried by multiconductor transmission lines, all in known fashion. While TR processors 518 of array 618 of FIG. 6 or array 718 of FIG. 7 have been described as being identical to TR processors 518 of column arrays 616 of FIG. 6 or 716 of FIG. 7, they need not be totally identical, specifically in terms of gain and output power.
The radar system as described in the Agrawal et al patent can be of any polarization, such as linear “vertical” or “horizontal,” or right or left “circular.” Those skilled in the art know that the terms “vertical” and “horizontal” are often applied to identify mutually orthogonal linear polarizations, regardless of the actual orientation of the electric field. Similarly, those skilled in the art know that true circular polarization is only a goal, and the best that can be achieved in practice is elliptical. Some radar systems applications, such as weather radar, require transmission of a linearly polarized signal, and simultaneous determination of the polarization characteristics of the return signal. The simultaneous determination of the polarization characteristics of the return signal, in turn, requires simultaneous measurement of two mutually orthogonal return or receive signals. In order to receive and process first and second mutually orthogonal “components” of the return signal, the radar system must include an array of antenna elements responsive to the first of the two polarization components, and another array of antenna elements responsive to the second polarization component, orthogonal to the first. The antenna elements of each of these differently responsive arrays may be co-located (in as much as possible) so that the antenna aperture is not doubled, but each of the two differently responsive receive arrays must have its own beamformers. It will be appreciated that the complexity and cost of providing capability for polarization discrimination will be substantial.
Improved or alternative polarization responsive receive array arrangements are desired.